When Microsoft discovers a bug in a product, whether it's Windows, Visual Studio, or some other product, they typically fix it. And once that fix is tested and working, the question becomes how to deploy it. Sure, you can put the corrected code into the next version of the product, but that might be years away. There are service packs, but they are pretty far apart, and for good reason. So the first deployment vehicle is the hotfix or QFE (Quick Fix Engineering.) These are typically available only from support and only after you've convinced support you're facing the problem that the hotfix takes care of. (At least, that's the theory. Since a hotfix is a single file, people can and do share them although I suspect you're not supposed to.)

Now there's a new pilot program underway where some hotfixes are being made public, so that you don't have to wait for a service pack. Of course you should approach this with caution, but it can make investigating those hard problems a little smoother. At the moment I see seven hotfixes there: three are for Visual C++.